@Html.TextBoxFor(x => x.SomeFieldDateTime, "{0:dd.MM.yyyy}"))

 internal static string FormatValueInternal(object value, string format)
        {
            if (value == null)
            {
                return String.Empty;
            }

            if (String.IsNullOrEmpty(format))
            {
                return Convert.ToString(value, CultureInfo.CurrentCulture);
            }
            else
            {
                return String.Format(CultureInfo.CurrentCulture, format, value);
            }
        }

public class TestClass
    {
        public string Msg { get; set; }
        public DateTime? MyDate { get; set; }
    }

@model testDateTime.Models.TestClass

@{
    ViewBag.Title = "title";
}

<h2>title</h2>

@using (Html.BeginForm("IndexPost", "Home", FormMethod.Post))
{
    @Html.TextBoxFor(x => x.Msg)
    

    @Html.TextBoxFor(x => x.MyDate, "{0:dd.MM.yyyy}")
    

    <input type="submit" value="Do" />
}

public class HomeController : Controller
    {
        public ActionResult Index(TestClass model)
        {
            if (model.MyDate == null)
                model.MyDate = DateTime.Now;

            if (model.Msg == null)
                model.Msg = "gdfgdgdg";

            return View(model);

        }

        public ActionResult IndexPost(TestClass model)
        {
            return RedirectToAction("Index", model);
        }
    }

вот код switch который рисует в TextBox
 default:
                    string attemptedValue = (string)htmlHelper.GetModelStateValue(fullName, typeof(string));
                    tagBuilder.MergeAttribute("value", attemptedValue ?? ((useViewData) ? htmlHelper.EvalString(fullName, format) : valueParameter), isExplicitValue);
                    break;

string valueParameter = htmlHelper.FormatValue(value, format);